The invention is a method of removing reactive metal from at least a portion of a metal-coated hydrocarbon conversion reactor system, so that the reactive metal does not deactivate the hydrocarbon conversion catalyst. It is especially applicable to catalytic reforming processes using halided catalysts.
Platinum L-zeolite catalysts for low-sulfur reforming were invented in the early 1980""s. After about 10 years of intensive effort, and much research, low sulfur reforming was commercialized in the early 1990""s. Progress toward commercialization required many discoveries. Two key discoveries were the criticality of ultra-low sulfur levels in the feed, and the impact of these ultra-low sulfur levels on reactor metallurgy, i.e., the discovery of the need to prevent coking, carburization and metal dusting. A preferred way to prevent coking, carburization and metal dusting utilizes a metal protective layer, especially one comprising tin.
While commercialization of ultra-low sulfur reforming was being pursued, a second generation of sulfur-sensitive platinum L-zeolite catalysts were being developed. These new catalysts are halided. They allow operations at higher severity, tolerate a wide range of hydrocarbon feeds, have high activity and long life.
Recent attempts to utilize this second generation of catalysts for ultra-low sulfur reforming resulted in an unexpected and undesired reduction in catalyst activity. After much research and experimentation, it was discovered that the catalyst had been partially poisoned by the metal of the protective layer specifically by tin; which had been used to prevent carburization and metal dusting of the reactor system surfaces. Somehow, some of this tin had migrated and deposited on the catalyst. In contrast, when conventional platinum L-zeolite catalysts are used for ultra-low sulfur reforming in a tin-coated reactor system, neither tin migration nor catalyst deactivation due to tin migration are observed. The cause of these problems has now been traced to low levels of volatile hydrogen halides that, under certain conditions, evolve from the catalysts themselves. These halides interact with reactive tin and can deactivate the catalyst.
Therefore, one object of the present invention is to reduce catalyst deactivation by metals derived from a metal-coated reactor system. Another object of the invention is to reduce catalyst contamination from a freshly metal-coated reactor system which would otherwise result in catalyst deactivation. This new process will also improve the reproducibility of catalytic operations, since catalyst activity and life can be better predicted.
The use of metal coatings and metal protective layers, especially tin protective layers, in hydrocarbon conversion processes is known. These layers provide improved resistance to coking, carburization and metal dusting, especially under ultra-low sulfur conditions. For example, Heyse et al., in WO 92/1856 coat steel reactor systems to be used for platinum L-zeolite reforming with metal coatings, including tin. See also U.S. Pat. Nos. 5,405,525 and 5,413,700 to Heyse et al. Metal-coated reactor systems are also known for preventing carburization, coking and metal dusting in dehydrogenation and hydrodealkylation processes conducted under low sulfur conditions; see Heyse et al., in U.S. Pat. No. 5,406,014 and WO 94/15896. In the ""014 patent, Example 3 shows the interaction of a stannided coupon with hydrocarbons, methyl chloride and hydrogen at 1000 and 1200xc2x0 F. The coupon was stable to methyl chloride concentrations of 1000 ppm at 1000xc2x0 F., showing that the tin coating is stable to halogens at reforming temperatures.
The use of catalysts treated with halogen-containing compounds for catalytic reforming is also known. See, for example U.S. Pat. No. 5,091,351 to Murakawa et al. Murakawa prepares a Pt L-zeolite catalyst and then treats it with a halogen-containing compound. The resulting catalyst has a desirably long catalyst life and is useful for preparing aromatic hydrocarbons such as benzene, toluene and xylenes from C6-C8 aliphatic hydrocarbons in high yield. Other patents that disclose halided L-zeolite catalysts include U.S. Pat. Nos. 4,681,865, 4,761,512 and 5,073,652 to Katsuno et al.; U.S. Pat. Nos. 5,196,631 and 5,260,238 to Murakawa et al.; and EP 498,182 (A).
None of these patents or patent applications disclose any problems associated with the metal-coated reactor systems. They neither teach the desirability nor the need for removing metal from the reactor system, especially not prior to catalyst loading or prior to hydrocarbon processing.
Indeed, the art teaches the advantages of combining one of the preferred coating metalsxe2x80x94tinxe2x80x94with a reforming catalyst, specifically with a platinum L-zeolite catalyst. U.S. Pat. No. 5,279,998 to Mulaskey et al., teaches that activity and fouling rate improvements are associated with treating the exterior of the platinum L-zeolite catalyst with metallic tin particles having an average particle size of between 1 and 5 microns (tin dust). For example, Table I of the Mulaskey patent shows improved catalyst performance when metallic tin dust is combined with a platinum L-zeolite catalyst that has been treated with fluoride according to the process of U.S. Pat. No. 4,681,865.
In light of the above teachings, we were surprised to find a decrease in catalyst activity upon reforming in a freshly tin-coated reactor system using a halided platinum L-zeolite catalyst. (See Example 5 below.)
Tin-coated steels are known to be useful for a variety of purposes. For example, surface coating compositions, known as stop-offs or resists, are temporarily applied to portions of a steel tool surface to shield them during case hardening. For example, in U.S. Pat. No. 5,110,854 to Ratliff the stop-off is a water-based alkyd resin containing tin and titanium dioxide.
It is also known that reacting tin with steel at elevated temperatures results in coated steels having surface iron stannides. Aside from hydrocarbon processing, as discussed above, coated steels have been used in applications where steels with hard and/or corrosion resistant surfaces are desired. For example, Caubert in U.S. Pat. No. 3,890,686 describes preparing mechanical parts having coatings consisting of three iron stannides to increase the resistance of these parts to seizing and surface wearing. In Example 2, a piece of coated steel is prepared by heating the steel to 1060xc2x0 F. in the presence of tin chloride (SnCl2) and hydrogenated nitrogen for 1.5 hours.
It is also known to treat tin-coated steels to further modify their properties. For example, Galland et al., in U.S. Pat. No. 4,015,950 teach that hot dipping stainless steel into molten tin results in two intermetallic stannide layers, an outer FeSn layer and inner layer which comprises a mixture of Fe (Cr,Ni,Sn) and FeSn2. The inner layer has a greater hardness. They teach that the outer layer can be removed by grinding, by reacting with 35% nitric acid containing a polyamine, or by electrochemical means, leaving behind the harder and more corrosion resistant inner layer.
Another example where tin-coated steel is modified is Carey II, et al., in U.S. Pat. No. 5,397,652. Here, tin-coated stainless steels are taught as roofing materials and siding, especially for use in marine or saline environments. Carey II, et al. teach that hot-dipping stainless steel into molten tin results in a bonded tin coating and an underlying intermetallic alloy of chromium-iron-tin. They teach treating the coated steel with an oxidizing solution (aqueous nitric acid) to obtain a uniformly colored stainless steel. The nitric acid preferentially reacts with the bonded tin coating leaving behind the uniformly colored intermetallic alloy. None of these patents on coated steels are concerned with hydrocarbon conversion processing.
None of the art described above is concerned with the problems associated with reactive metals derived from metal coatings, such as tin coatings, nor with the effect of these reactive metals on catalysts, especially platinum L-zeolite reforming catalysts.
We have discovered that there are problems associated with using metal-coated reactor systemsxe2x80x94especially freshly-coated systemsxe2x80x94in the presence of certain catalysts, and we have discovered the cause of and solutions for these problems. Thus, one object of the present invention is to reduce catalyst contamination from a freshly metal-coated reactor system. Another object of the invention is to ensure that catalyst contamination is avoided, for example when replacing a conventional catalyst with a halided catalyst.
In one embodiment, the invention is a method of removing reactive metal from at least a portion of a metal-coated reactor system that is used for converting chemicals, especially hydrocarbons. The method comprises contacting at least a portion of a reactor system containing reactive metal with a getter to produce movable metal, and fixating the movable metal, the getter, or both. The getter reacts with the reactive metal (derived from the coating metal) facilitating its removal from the reactor system. Preferably the movable metal and the getter are both fixated, for example by trapping using a solid sorbent.
Preferred metal coatings are those prepared from tin-, germanium-, antimony-, and aluminum-containing compositions. More preferably, the reactive metal comprises a tin-containing composition including elemental tin, tin compounds or tin alloys.
Preferably the getter is prepared from a gaseous halogen-containing compound; more preferably the getter comprises a hydrogen halide, especially HCl prepared in-situ. In a preferred embodiment, a conversion catalyst is then loaded into the reactors after the reactive metal is removed, and conversion operations begin.
In another embodiment, the invention is a method of removing reactive tin from at least a portion of a reactor system having freshly-stannided surfaces. The method comprises the steps of:
a) applying a tin plating, paint, cladding or other coating to a iron-containing base substrate portion of a reactor system;
b) heating the coated substrate at temperatures greater than 800xc2x0 F., preferably in the presence of hydrogen to produce a reactor system having freshly-stannided surfaces and which contains reactive tin;
c) removing at least a portion of the reactive tin from the reactor system by contacting the reactive tin with a getter to produce movable tin; and
d) sorbing or reacting the movable tin.
Preferably, the reactive tin is removed by contacting the surface portion with a gaseous halogen-containing compound, such as HCl. In general, the contacting is done at temperatures and flow rates sufficient to transport a significant amount of the movable tin out of the reactor and furnace tubes and onto a sorbent. Here again it is preferred that the method be conducted prior to catalyst loading.
In yet another embodiment, the invention is a method for reducing catalyst contamination from a metal which was used to coat a reactor system. The method comprises contacting a metal-coated reactor system prior to catalyst loading with a getter, preferably a gaseous halogen-containing compound, to produce movable metal; and removing and fixating at least a portion of the movable metal from the reactor system. The conversion catalyst is then loaded into the reactor system, and conversion operations begin with feed being converted to product in the reactor system. This method is preferably applied to a freshly-coated reactor system.
In yet another embodiment, the invention is a catalytic reforming process. The process comprises removing reactive tin from a tin-coated reforming reactor system by contacting a tin-coated reactor system with a halogen-containing compound to produce movable tin; mobilizing and sorbing the movable tin; loading a halided Pt L-zeolite catalyst into the reactor system; and reforming hydrocarbons to aromatics.
Among other factors, this invention is based on our observation that halided Pt L-zeolite catalysts are partially deactivated during the start-up phase of a catalytic reforming process, especially when the start-up is done in a freshly tin-coated reactor. This is in contrast to what is observed with conventional Pt L-zeolite catalysts (which are not halided); here catalyst deactivation due to a tin coating has not been noted.
The art appears to be totally silent about the presence of reactive metal in metal-coated hydrocarbon conversion reactor systems. Moreover, the art has not appreciated the need or desire to remove this reactive metal prior to catalyst loading, especially prior to loading halided catalysts. We have found that tin-coated reactor systems, especially those with freshly prepared tin intermetallics, can lose tin from the tin-coated surfaces when contacted with halogen-containing gases, for example, during the start-up of a reforming process using a halided Pt L-zeolite catalyst which evolves acid halides, including HCl. This metal loss results in tin depositing on the catalyst and reduced catalyst activity.
However, we have observed that after several start-up cycles, catalyst activity stabilizes without significant further decline. Thus, we believe that a reactive tin is present in the freshly-coated reactors. When contacted with hydrogen halides (e.g., HCl and/or HF), this tin is unexpectedly mobilized and deactivates the platinum catalyst. Based on these discoveries, we have developed simple, inexpensive procedures that quickly and efficiently remove reactive tin from tin-coated reactor systems, preferably prior to catalyst loading. When the catalyst is then loaded into the reactors and hydrocarbon processing begins, the catalyst experiences little or no deactivation from the tin coating.